Aero engines and gas turbines, as national strategic equipment, have a profound impact on national defense security, energy security, and technological innovation. With technological advancements and the pursuit for enhanced efficiency, the combustion chamber temperatures of aero engines and gas turbines continue to rise, subjecting hot-section components to severe challenges of high temperature, oxidation and corrosion. Surface coating technology is a key solution to increase the service temperature and corrosion resistance of components, among which thermal barrier coatings, environmental barrier coatings, and integrated thermal/environmental barrier coatings are the mainstream protective coating systems. Regardless of the various coating systems, a bond coat is required between the top coat and the substrate (superalloy/ceramic matrix composite). This layer, designed with a matched coefficient of thermal expansion, effectively alleviates thermal stress and reduces the risk of coating cracking; its active elements can also react with oxygen to form a dense oxide layer, preventing substrate oxidation and thus determining the overall lifespan of the coating system. This paper systematically summarizes the material characteristics, technical advantages, and limitations of bond coats for different coating systems, and reviews the research progress. It introduces the preparation techniques for bond coats, analyzes their suitability in service environments, and identifies current technical bottlenecks in research. Finally, it discusses the future development trends of bond coat technology.

Ultrahigh-temperature ceramics (UHTCs) have a unique combination of high melting points, high strengths, and high chemical stabilities, which makes them unique materials for a wide range of ultrahigh-temperature (> 2000 °C) applications. Herein, we first report a novel highly porous dual-phase high-entropy UHTCs material composed of a high-entropy boride (HEB) phase and a high-entropy carbide (HEC) phase, which was fabricated via foam-gelcasting-freeze drying technology and high-temperature sintering with mixed borides and carbides as raw materials. The as-fabricated samples have a uniform pore structure and a firm skeleton that consists of random alternating distributions of HEB and HEC particles. The porous dual-phase high-entropy UHTCs samples have ultrahigh porosities of 96.4%–90.1%, low densities of 0.31–0.87 g/cm³, high strengths of 0.45–4.17 MPa and low thermal conductivities of 0.202–0.281 W/(m·K), as well as better oxidation resistance than single-phase HEC. The present results highlight the potential of as-prepared porous dual-phase high-entropy UHTCs as promising materials for ultrahigh-temperature thermal insulation applications.